English: This graphic shows how oddly ‘light’ carbon monoxide forms in Mars’ atmosphere, based on new observations from ESA’s ExoMars Trace Gas Orbiter (TGO).
The TGO observations show that a process at play in Mars’ atmosphere forms carbon monoxide (CO) containing less ‘heavy’ carbon than we would expect. The finding is consistent with the idea that a combination of sunlight and complex chemistry, rather than life, gave rise to the carbon-based compounds (‘organic matter’) we see on the martian surface.
Carbon dioxide (CO2) in Mars’ atmosphere is broken apart by sunlight, forming oxygen and CO. ‘Light’ CO2 is more easily broken down by sunlight than ‘heavy’ CO2, causing more ‘light’ CO to accumulate in the martian atmosphere. This suggests that the CO may then play a role in forming organic matter on Mars' surface, as this material was recently found to be equally enriched in ‘light’ carbon.
‘Light’ carbon, carbon-12, accounts for most of the carbon in the Solar System; ‘heavy’ carbon, or the isotope carbon-13, contains an extra neutron. Both are present in Mars' atmosphere. Measuring the relative amounts of each at Mars can reveal a great deal about an environment’s past and present as many short- and longer-term processes affect this ratio. For instance, on Earth, photosynthesis uses more ‘light’ than ‘heavy’ carbon, so plants and animals are often enriched in carbon-12. Excitingly, NASA's Curiosity rover has found carbon-12-enriched material on Mars’ surface; while martian biology is a possible cause, the new TGO results point in a different direction.
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Alt-text: Infographic showing how oddly ‘light’ carbon monoxide forms in Mars' atmosphere. It shows carbon dioxide molecules being split by sunlight to form carbon monoxide and oxygen, with an illustration of Mars' red-hued surface below.